Circuit breakers are electromagnetically controlled to react upon a critical magnitude of the current so as to open the electrical contacts interconnecting the power supply and the load, whenever there is an excess of current flowing through the load, or derived from the power supply. In the process, two alternative steps may be taken by the circuit breaker which correspond to different degrees of urgency. One is to cause a response to an overload which will trigger a predetermined succession of mechanical steps for disconnecting the movable contact from the stationary contact. The other is to ensure a direct and immediate disconnection between the contacts if the size of the current indicates a condition serious enough, such as a short-circuit, to require immediate action.
U.S. Pat. Nos. 3,863,042 and 4,001,743 show electromagnetic actuation of a linkage mechanism kicked by the coil armature for tripping a circuit breaker. Slow operation is generally initiated by a bimetal, whereas instant command to separate the movable contact from the fixed contact of the circuit breaker is obtained with a kicker arm stricken by the electromagnetic coil armature and striking the movable contact away, as shown in U.S. Pat. No. 4,654,614. U.S. Pat. No. 4,503,408 illustrates trip bar rotation for actuating a toggle mechanism to separate the contacts.
Where a circuit breaker is used with a motor-starter, there is a need to foresee two possibilities:
1) the immediate control of the circuit breaker upon a serious overload, such as a voltage surge or a short-circuit, and
2) circuit breaker operation upon an over-current which should not last for the motor because of heating, despite the much lower level of the critical current.
If the trip bar is set for actuation at the lower critical level, for quick action it will work only at the much higher critical level required in the second situation. In this respect, U.S. Pat. No. 4,042,895 shows a combination motor-starter circuit breaker wherein the three functions of motor starting, circuit interruption and current limiting have been separately provided for. U.S. Pat. No 4,697,163 shows an electromagnetic coil with a plunger action determined by an airgap selected for the rest position, so as to establish a delay under overcurrent which will be caused by the biasing spring before and the striking arm can hit the trip bar of the circuit breaker and release the toggle mechanism to separate the circuit breaker contacts.
It is desirable to combine more closely in a circuit breaker the separate functions of:
1) responding immediately to an excessive current or voltage, and
2) of reacting to an overload as can be experienced when an AC motor is working with a load under the AC supply lines.
Such closer combination should be so achieved as to simplify the mechanical and the electromagnetic design within the housing of a circuit breaker, while allowing to perform any of the several required functions to be exerted upon the movable contact in response to various possible load conditions.